The transmissivity of light through an optical fiber is highly dependent on the properties of the coatings applied to the fiber. The coatings typically include a primary coating and a secondary coating, where the secondary coating surrounds the primary coating and the primary coating contacts the glass waveguide (core+cladding) portion of the fiber. The secondary coating is a harder material (higher Young's modulus) than the primary coating and is designed to protect the glass waveguide from damage caused by abrasion or external forces that arise during processing and handling of the fiber. The primary coating is a softer material (low Young's modulus) and is designed to buffer or dissipates stresses that result from forces applied to the outer surface of the secondary coating. Dissipation of stresses within the primary layer attenuates the stress and minimizes the stress that reaches the glass waveguide. The primary coating is especially important in dissipating stresses that arise when the fiber is bent. The bending stresses transmitted to the glass waveguide on the fiber needs to be minimized because bending stresses create local perturbations in the refractive index profile of the glass waveguide. The local refractive index perturbations lead to intensity losses for the light transmitted through the waveguide. By dissipating stresses, the primary coating minimizes bend-induced intensity losses.
To minimize bending losses, it is desirable to develop primary coating materials with increasingly lower Young's moduli. Coating materials with a Young's modulus below 1 MPa are preferred. As the Young's modulus of the primary coating is reduced, however, the primary coating is more susceptible to damage in the fiber manufacturing process or during fiber installation or deployment. Thermal and mechanical stresses that arise during the fiber coating process or during post-manufacture fiber handling and configuration processes (e.g. stripping, cabling and connecting operations) may lead to the formation of defects in the primary coating. The defect formation in the primary coating becomes more problematic as the Young's modulus of the primary coating material decreases. There is a need for a primary coating material that has a low Young's modulus and yet is resistant to stress-induced defect formation during fiber manufacture and handling.